Split screen

Split screen is the combination of two or more scenes that are filmed separately and then juxtaposed against eachother which then appear simultaneously in the same frame.

Typically, a line cuts through the middle of the screen to convey simultaneous action. Filmmakers use this device to tell various stories at the same time, which can essentialy build tension from multiple points of view, but also show the effect the past has on the present, and so on.